1. Field of the Invention
This invention relates generally to a method of manufacturing spin transfer magnetic-random-access memory (MRAM) having magnetoresistive elements as basic memory cells, more particularly to a method of fabricating magnetoresistive elements having ultra small dimensions by dual photo exposures and etching of hard masks.
2. Description of the Related Art
In recent years, magnetic random access memories (hereinafter referred to as MRAMs) using the magnetoresistive effect of ferromagnetic tunnel junctions (also called MTJs) have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can cope with high-speed reading and writing, large capacities, and low-power-consumption operations. A ferromagnetic tunnel junction has a three-layer stack structure formed by stacking a recording layer having a changeable magnetization direction, an insulating spacing layer, and a fixed layer that is located on the opposite side from the recording layer and maintains a predetermined magnetization direction.
There has been a known technique for achieving a high MR ratio in a magnetoresistive element by forming a crystallization acceleration film that accelerates crystallization and is in contact with an interfacial magnetic film having an amorphous structure. As the crystallization acceleration film is formed, crystallization is accelerated from the tunnel barrier layer side, and the interfaces with the tunnel barrier layer and the interfacial magnetic film are matched to each other. By using this technique, a high MR ratio can be achieved.
As a write method to be used in such magnetoresistive elements, there has been suggested a write method (spin torque transfer switching technique) using spin momentum transfers. According to this method, the magnetization direction of a recording layer is reversed by applying a spin-polarized current to the magnetoresistive element. Furthermore, as the volume of the magnetic layer forming the recording layer is smaller, the injected spin-polarized current to write or switch can be also smaller. Accordingly, this method is expected to be a write method that can achieve both device miniaturization and lower currents.
In the mean time, since the switching current requirements reduce with decreasing MTJ element dimensions, STT-MRAM has the potential to scale nicely at the most advanced technology nodes. However, patterning of small MTJ element may lead to increasing variability in MTJ resistance and sustaining relatively high switching current or recording voltage variation in a STT-MRAM; accordingly a degradation of MRAM performance would occur. Due to the limitation (such as UV light source and photo-resist thickness) of the current photolithography technology, it is also difficult to form ultra-small photo-resist pillar pattern. Once a cell dimension is getting too small, the photo-resist pillars will not be strong enough to support themselves and bend or tilt; accordingly causing a variation in magnetoresistive element dimensions. More seriously, some photo-resist pillars may collapse before etching; thereby defects are generated.
Thus, it is desirable to pattern STT-MRAM elements into ultra small dimensions having a good uniformity and minimum impact on MTJ magnetic properties by a manufacturing method that realizes high yield, highly-accurate reading, highly-reliable recording and low power consumption while suppressing destruction and reduction of life of MTJ memory device due to recording in a nonvolatile memory that performs recording resistance changes, and maintaining a high thermal factor for a good data retention.